Amino acids play a very important role not only as a substrate for protein synthesis but also as a precursor in gluconeogenesis and in biosynthesis of many biomolecules such as porphyrin, purine and pyrimidine.
Most of such a biosynthesis reaction is carried out in cells and, therefore, in the cells, various proteins which are generally called amino acid transporters for incorporating amino acid into the cells from outside of the cells are contained in cell membrane.
The amino acid transporter not only functions for supplying amino acids to each cell but also is incorporated into tissues playing a role of an epithelial transport of amino acids in small intestine and renal tubule and a resorption of neurotransmitter in nervous tissue whereby it is oriented at an important position for the expression of a specific function of tissues.
With regard to an amino acid transport mechanism (an amino acid transport system mediated by amino acid transporter), its identification and classification have been carried out using cultured cells and cell membrane samples since about 1960's and, reflecting the diversity of amino acid molecules, there have been identified amino acid transport systems mediated by various amino acid transporters having different substrate specificity (Physiol. Rev. Vol. 70, p. 43-77, 1990).
However, such a transport system does not independently function to each amino acid but each amino acid bears an intracellular transport for more than one amino acids using the more than one amino acids as substrates.
Amino acids are classified into basic amino acids (diamino/monocarboxylic acids) having positive charge, acidic amino acids (monoamino/dicarboxylic acids) having negative charge and neutral amino acids (monoamino/monocarboxylic acids; or those excluding basic amino acids or acidic amino acids). Because of such a charge of amino acid, when a neutral amino acid or an acidic amino acid having negative charge, for example, is transported into cells having negative electric potential against the concentration gradient, it is necessary to carry out an active transport associated with some energy consumption.
From such a viewpoint, in amino acid transporters, that which shows a dependency on sodium (Na+) and that which shows an independency on Na+ are present like in the case of a sugar transport system. The Na+-dependent transporter has a big concentrating ability since it is able to transport the amino acid against the concentration gradient by coupling amino acid transport with Na+ transport and, therefore, it plays an important role in the site in a living body where formation of a big concentration difference mediated by cell membrane is requested (Annual Rev. Kidney “Structure and Function of Kidney-Specific Organic Solute Transporters”, p. 91-100, 1995, published by Chugai Igakusha). Such a Na+-dependent transporter can be further classified into two families, i.e. an Na+/Cl−-dependent transporter family and an Na+/K−-dependent transporter family (Annual Rev. Neurosci., Vol. 16, p. 73-93, 1993 and FASEB J., Vol. 7, p. 1450-1459, 1993).
Further, in combination of such a charge property, amino acid transporters can be classified, in view of the substrate specificity, into molecule where basic amino acid (diamino/monocarboxylic acid) is a substrate, molecule where acidic amino acid (monoamino/dicarboxylic acid) is a substrate and molecule where neutral amino acid (monoamino/monocarboxylic acid; or that excluding basic amino acid or acidic amino acid) is a substrate.
It has been known that, for example, a basic amino acid having amino group or imidazole group on the side chain such as arginine, lysine and histidine (basic amino acid which is nearly neutral) is transported mostly by an Na+-independent amino acid transporter y+ (J. Membrane Biol., Vol. 66, p. 213-225, 1982). It has been known that an acidic amino acid having carboxyl group on the side chain such as glutamic acid and aspartic acid is transported by an Na+-dependent amino acid transporter X−A,G (Biochim. Biophys. Acta, Vol. 732, p. 24-31, 1983). In the case of transport of a neutral amino acid to which many amino acids belong, it has been known that an Na+-independent amino acid transporter L (Ann. Rev. Physiol., Vol. 46, p. 417-433, 1984) and Na+-dependent amino acid transporters A and ASC (Ann. Rev. Physiol., Vol. 46, p. 417-433, 1984 and J. Membrane Biol., Vol. 52, p. 83-92, 1980) play an important role (Physiol. Rev., Vol. 70, p. 43-77, 1990 and Saishin Igaku, Vol. 50, p. 1997-2004, 1995).
As mentioned already, amino acids play a very important role as materials in biosynthesis of various biocomponents taking place in cells and, therefore, it is presumed that abnormal transport of the amino acid into cell participates in various symptoms.
It has been known from the studies up to now that the symptoms in which abnormal transport mechanism of the amino acid into cells are participated are aminoaciduia where disorder of amino acid resorption from renal tubule occurs and amyotrophic lateral sclerosis, etc. in which disorder of glutamic acid incorporation and nerve cell death are participated (Annual Rev. Kidney “Structure and Function of Kidney-Specific Organic Solute Transporters”, p. 91-100, 1995, published by Chugai Igakusha; Saishin Igaku, Vol. 50, p. 1997-2004, 1995; and Saishin Igaku, Vol. 51, p. 64-70; 1996).
Amino acid transporters play an essential and very important role in incorporation of amino acids necessary for generation, differentiation, proliferation and maintenance of all cells and, therefore, they are believed to participate not only in the above-mentioned symptoms but also in onset of many other symptoms. In addition, when the indispensability of amino acid transporter in living body is taken into consideration, it is hardly concluded that incorporation of various amino acids is not mediated by several already-identified transporters only but it is believed that many other unknown amino acid transporters will be present.
Identification of such unknown amino acid transporters which play a role essential for existence and maintenance of cells, tissues, organs and living body has a possibility of clarification of causes for onset of various diseases for which the causes have not been clarified yet. In addition, if the relation between such amino acid transporters and various diseases is made clear, an effective treatment of such diseases will become possible by regulation of biological function or expression of the amino acid transporters. Accordingly, it is a pressing need to identify new amino acid transporter and to clarify the relation between the transporter molecule and a symptom.
However, in spite of medical and social needs as such, it is a current state that there has been little progress in identification of an amino acid transporter and clarification of an amino acid transport mechanism.
Thus, in order to identify an amino acid transporter molecule, it is necessary to purify the molecule and, in order to analyze the activity of the purified substance, it is necessary to reconstitute the purified substance to cell membrane so that an amino acid transport activity is regenerated. However, an amino acid transporter molecule has a relatively little expression amount as a membrane protein and has a relatively small regenerating efficiency and, therefore, there is a difficulty in the technique in the identification of new molecules.
In addition, identification of an amino acid transporter which is specifically expressed in abnormal cells directly participating in the symptom such as cancer cells (tumor cells) and plays a role of supplying an amino acid to the abnormal cells has a very important significance in the clarification of existence and proliferation of such symptom-related cells and also in the development of therapeutic methods for cancer, etc. However, an amino acid transporter is inherently a molecule essential for the existence of normal cells and is believed to be present in a wide range of cell species and, accordingly, it is not easy to identify an amino acid transporter molecule which is expressed specifically in such abnormal cells.
As a neutral amino acid transporter, ASCT1 and ASCT2 have been cloned as sodium-dependent transporters (Kanai, Curr. Opin. Cell Biol., 9, 565 (1997)). However, the main substrates thereof are alanine, serine, cysteine, threonine and glutamine and their substrate specificity is different from a neutral amino acid transporting system L. Further, glycine transporter and proline transporter have been cloned but their substrate specificity is different from neutral amino acid transport system L (Amara and Kuhar, Annu. Rev. Neurosci., 16, 73 (1993)).
Although not a transporter per se, cDNAs of rBAT and 4F2hc which are a type II membrane glycoprotein having only one transmembrane structure believed to be an activating factor for an amino acid transporter have been cloned and it has been known that, when they are expressed in oocytes of Xenopus laevis, uptake of a basic amino acid is activated together with that of a neutral amino acid (Palacin, J. Exp. Biol., 196, 123 (1994)).
Accordingly, it is an effective key for providing a therapeutic method for the symptom and disease that an amino acid transporter molecule which has not been identified yet and is specifically expressed in abnormal cells deeply associated with such a symptom is identified and that the relation between the molecule and existence/proliferation of the abnormal cells is clarified.
Thus, when the biological activity of the amino acid transporter molecule or the expression of the molecule is controlled, it is now possible to treat the diseases.
The present inventors have paid their attention to the already-known cell membrane surface molecule 4F2 (CD98) which is believed to be essential for the proliferation of tumor cells in order to investigate a novel amino acid transporter which is specifically expressed in such symptom-related abnormal cells or, particularly, tumor cells and have succeeded in identifying a novel amino acid transporter molecule named LAT1 (L-type amino acid transporter-1) which is particularly significantly expressed in tumor cells as compared with the expression in normal cells.
Thus, for a quick cell division and continuous growth and proliferation, the tumor cells are to be provided with nutrients such as amino acids and saccharides thereinto and it is believed that such a providing is carried out by means of an up-regulation of an amino acid transporter which is specific to the nutrients (Physiol. Rev., Vol. 70, p. 43-77, 1990). For growth, proliferation and maintenance of the tumor cells, a protein biosynthesis is to be carried out in the cells and, therefore, incorporation of the essential amino acids into the cells (transport from outside of the cells to inside of the cells) is particularly important.
From the studies up to now, it has been believed that, for the proliferation of tumor cells, a known cell membrane surface antigen named 4F2 (CD98) classified into a type II membrane glycoprotein believed to have a function of activating the amino acid transporter which has not been identified yet will play an important role (J. Immunol., Vol. 126, p. 1409-1414, 1981; J. Immunol., Vol. 129, p. 623-628, 1982; Proc. Natl. Acad. Sci. USA., Vol. 84, p. 6526-6530, 1987; Cancer Res., Vol. 46, p. 1478-1484, 1986; J. Biol. Chem., Vol. 267, p. 15285-15288, 1992; Proc. Natl. Acad Sci USA, Vol. 89, p. 5606-5610, 1992; Biochem. J., Vol. 324, p. 535-541, 1997; and J. Exp. Biol., Vol. 196, p. 123-137, 1994).
Under such circumstances, the present inventors have carried out an intensive investigation for identification of human tumor cell membrane surface molecule which conjugates or interacts with a 4F2 molecule and found a gene coding for a novel amino acid transporter LAT1 having the following characteristics whereupon the present invention has been accomplished.